1. Field of the Invention
The present invention relates to a buck—boost power factor correction circuit, and more particularly to a power factor correction circuit able to improve efficiency of increasing and decreasing voltage.
2. Description of Related Art
Power factor in terms of an AC electrical power system relates to real power and apparent power, and is defined as the ratio between real power and apparent power in a circuit. Generally, power factor is used to evaluate amount of useful electric power. Higher power factor means higher efficiency in electrical power system. A power factor correction device in power supply is used to adjust the input timing and waveform of AC current in accordance with the shape of DC voltage waveform, and the power factor is close to 1, then the consuming energy in electrical power system can be controlled according to request standard, so avoiding unnecessary cost to industrial utilities.
Low power factor has low efficiency, costs more energy and draws more current than a load with high power factor for the same amount of useful power transferred. Electric supplier provides not only real power but also extra work for low power factor without correction, and consuming energy in electrical transferring utilities including generator, motor, transfer tool, or even cable increase to compensate low power factor. Power factor calibration is one of the power factor correction devices being widely used in electrical utilities to improve self energy efficiency.
The circuit in power factor calibration can be single-phase power factor correction circuit or three-phase power factor correction circuit. Please refer to FIG. 1, which demonstrating a power factor correction circuit formed by three single-phase power factor correction circuit. As shown in FIG. 1, single-phase power factor correction circuit increases conduction angle of bridge rectifier with LC filter, and reduces electrical harmonic to increase power factor. However, three single-phase power factor correction circuit affect each other during work, and the interference cannot be eliminated by adding shielded inductor, so the efficiency of circuit and the index related to total harmonic distortion of input current are decreased, thus, single-phase power factor correction circuit does not usually be used in electric high power.
Please refer to FIG. 2, which demonstrating a conventional three-phase power factor correction circuit with six switches. As shown in FIG. 2, a conventional three-phase power factor correction circuit is formed with six switches, three phase bridges are connected in parallel, each phase bridge has two switches on both sides, and the current in each phase bridge can be controlled by the switches. The sum of the current in three-phase power factor correction circuit is zero, so two phase bridges being controlled is enough for three-phase power factor correction circuit. In practical application, one phase bridge which has the voltage with large absolute value is fixed when other two are controlled simultaneously. The switch times can be regulated and controlled when one phase bridge is fixed, to reduce damage during switching. However, the switches used in three-phase power factor correction circuit require at least six, and complex circuit design with high cost, furthermore, operating switches need to be accurate and precisely, to avoid the risk of short circuit when two switch in one phase bridge turning on in the same time.
Input power to three-phase power factor correction circuit is a fixed value and relatively higher than single-phase power factor correction circuit. The capacitor with smaller capacity can be used in output of three-phase power factor correction circuit, to operate voltage dynamic respond adjustment. Due to switch operation need to be accurate and precisely, the three-phase power factor correction circuit is hard to control. In single-phase power factor correction circuit, power factor is 1 when the load is equal to a resistance. In three-phase power factor correction circuit, power factor is hardly being close to 1 even the load is equal to a resistance, because input current in three phase bridge of three-phase power factor correction circuit is hard to operate, so the sine wave is never performed simultaneously. Conventional skills try to improve and create typical three-phase power factor correction circuits, though, conventional skill requests plenty electrical elements, complex operation, with high voltage operation, so increasing cost.
The single-phase power factor correction circuit and the three-phase power factor correction circuit both have advantage, where single-phase power factor correction circuit has low cost, and three-phase power factor correction circuit can work in high voltage. So the research keeps finding the leverage between both advantages in different electrical circuit appliance.
Thus, for balancing between price and function, reducing element used in structure, keeping response rapidly, the inventor of the present invention based on years of practice experience in the related industry to conduct extensive researches and experiments, and finally developed a power supply with power factor correction circuit in accordance with the present invention to overcome the drawbacks of the prior art.